Pipelines composed of steel pipe are commonly used to transport a variety of liquids and gases. Depending on the application and the environment, the pipe may be used above the ground or it may be buried. In order to protect the pipe from abrasion, corrosion, and degradation due to exposure to sun, soil, moisture, and other elements, it is often necessary to coat the exterior of the pipe with a protective layer. The nature of this protective layer depends on the environment to which the pipe is exposed, but, for many applications, the protective layer is polymeric. The protective layer may comprise a polymer based tape which is coated with an adhesive layer and is then applied to the pipe, or it may comprise an epoxy which is electrostatically sprayed or painted onto the pipe. Another type of effective coating is a fusion bonded epoxy coating in which the epoxy powder is sprayed onto the pipe and is then cured to form a pinhole-free layer which is suitable for use up to about 60° C. Frequently the protective coating comprises a number of layers, for example an epoxy layer, which is immediately adjacent the pipe and an outer polymer layer such as polyethylene. Such coatings provide the advantages of the epoxy and the polymer and are useful up to a temperature of about 110° C.
Several manufacturers have introduced coated steel pipe, which is designed to withstand exposure temperatures of up to 110° C. Such pipe generally has a coating, which comprises an inner layer of fusion bonded epoxy, an optional adhesive layer, and an outer layer of polypropylene. These layers provide excellent protection to the steel pipe because they combine the excellent adhesion and cathodic disbanding resistance of the epoxy with the low moisture absorption, low moisture transmission, and abrasion resistance of polypropylene. The difficulty with such pipe is that there are currently no easily installable coatings for the pipe joints or welds that are created when pipe is repaired or retrofitted, for example, when two pieces of pipe are attached to one another. Therefore, when a joint or a weld must be made to the polypropylene-coated pipe, difficult and time-consuming procedures must be followed to ensure that the pipe joint or weld is adequately protected. For example, one technique requires preheating the weld region to about 240° C. by induction heating, then applying a layer of fusion bonded epoxy powder followed by a layer of polypropylene. To ensure adequate adhesion, the polypropylene sheet is heated with a torch and smoothed out with a spatula. The procedure, which requires a skilled craftsman, is especially difficult to carry out in the field where the conditions are not easily controlled.
One technique for supplying a protective coating layer to a pipe joint or weld in the field is by the use of a heat-recoverable polymeric article, that is, a heat-shrinkable polymeric article such as a sleeve, sheet, or tape, in combination with a heat-activatable sealant. The polymeric material of the article has been crosslinked during the production process so as to enhance the desired dimensional recovery. One method of producing a heat-recoverable article comprises shaping the polymeric material into the desired heat-stable form, subsequently crosslinking the polymeric material, heating the article to a temperature above the crystalline melting point of the polymer (or, for amorphous materials, the softening point of the polymer), deforming the article, and cooling the article while in the deformed state so that the deformed state of the article is retained. In use, because the deformed state of the article is heat-unstable, application of heat, by means of a torch or other heat source, will cause the article to assume its original heat-stable shape. For many applications, the article is designed to shrink down onto the substrate.
To enhance adhesion to the substrate, such heat-recoverable articles generally comprise a layer of a heat-activatable sealant which is positioned adjacent the substrate. When the article is heated, the temperature of the heat-activatable sealant rises above its activation temperature so that a strong bond is formed between the polymeric article and the substrate. Suitable sealants include pressure sensitive adhesives, hot melt adhesives and mastics. Good adhesion of the coating to the substrate is particularly important to avoid cathodic disbonding. Cathodic disbanding occurs as a result of the impressed electric current that is applied to the pipe to prevent corrosion of the iron in the steel pipe. Many adhesive compositions. which are used to bond a protective coating onto a pipe. are adversely affected by the impressed electric current. As a result, the bond weakens and the adhesive disbonds from the pipe, leaving segments of the pipe surface exposed to the corrosive conditions of the environment.
For pipe protection, conventional heat-recoverable articles are sleeves which comprise a backing layer, generally polyethylene or a copolymer comprising ethylene, in contact with a hot melt adhesive, which is often a polyethylene-based adhesive. The hot melt adhesive is selected to have adequate adhesion both to the backing layer and to the substrate. Most conventional polyethylene-based heat-recoverable sleeves do not adhere well to a polypropylene-coated pipe, and those sleeves which do have good adhesion cannot be used for pipelines which are exposed to high temperatures, for example more than 110° C., because they contain adhesives or mastics which soften and disbond from the pipe under soil stress and at high temperatures. Although there are a number of commercial polypropylenes and modified polypropylenes which can be used in adhesives which stick well to polypropylene-coated pipes, these materials do not have good adhesion to polyethylene backing layers. Furthermore, because polypropylene degrades when crosslinked by means of electron beam irradiation, a heat-recoverable sleeve comprising a polypropylene backing layer is not practical.
In general, as mentioned, steel pipes are coated with polymeric materials to prevent the steel from corrosion and to provide mechanical protection. Polypropylene coatings are desirable as a result of high temperature resistance, low moisture absorption, good abrasion resistance and exceptional resistance to mechanical damage. The polypropylene line coatings are available in two main forms, namely, three-layer polypropylene and foamed polypropylene. The three-layer polypropylene comprises an epoxy corrosion protection layer, a polypropylene copolymer tie-layer, and an outer polypropylene mechanical protection layer. The foamed polypropylene has the same sequence of layers as the three-layer polypropylene line coating, with the insertion of a foamed polypropylene between the polypropylene copolymer tie-layer and the outer polypropylene mechanical protection layer.
The polypropylene line coatings are applied in specialized coating factories. At the ends of the steel pipes, the coatings are cut back over a length of 150 mm±20 mm. Thereafter, the pipes are welded together in the field, leaving a portion of the steel pipe exposed. It is necessary to protect the exposed steel against corrosion.
It has been found that providing a field joint coating process that will bond well with the polypropylene coating applied at the factory is difficult. There are available proposed solutions to the problem, such as injected polypropylene, sprayed polypropylene, injected polyurethane, welded polypropylene and heat shrink sleeves. However, these techniques do not provide an easy, fast and inexpensive solution for the grit-weld joint of polypropylene coated pipelines. It would, therefore, be desirable to provide an easily installable heat-shrinkable sleeve that will be suitable for use at operating temperatures up to about 120° C.